This invention relates to the improved regioselective ambient temperature nitration of diphenyl compounds in the presence of nitrobenzene and, more particularly, to an ambient temperature nitration process, which employs nitrobenzene as a nitrating solvent, of diphenyl compounds in which each phenyl ring is selectively nitrated in the para position to form a di(4-nitrophenyl) compound. When the selective nitration process is combined with a novel regiopurification technique, essentially regiopure 2,2-di(4-nitrophenyl) carbonate can be made which may be decarboxylated and reduced to essentially regiopure di(4-nitrophenyl) ether. The regioselective nitration process taught here provides a convenient route to di(4-nitrophenyl) compounds generally, such as 2,2-di(4-nitrophenyl)propane and its hexafluoro analogue, compounds which can be reduced to the corresponding diamines to be used in polyamides, polyimides, and polyamide-imides.
The commercial preparation of substituted diphenyl compounds such as diphenyl ethers in which each ring is monosubstituted has become important because of the use of such materials, in particular, the diamino derivatives, in polyamides, polyimides, and polyamide-imides. Because of the multiplicity of regioisomers formable in the manufacture of such compounds and the difference in properties of materials made from the different isomers, it is of commercial importance to be able to cheaply produce highly regiopure compounds. In particular, it is important to be able to make highly regiopure 4,4'-compounds such as di(4-nitrophenyl) ether and 2,2-di(4-nitrophenyl)propane and its hexafluoro analogue since these compounds are easily reduced to their di(4-aminophenyl) analogues for use in the polymer industry to manufacture polyamides, etc.
A number of methods have been devised to synthesize highly regiopure dinitro and diamino compounds but the successful ones are carried out at low temperature or often begin with regiopure starting materials which have been made by costly and time-consuming procedures. See German Offenlegenschrift 2,549,036 (5 May 1977) which teaches a 93% selectivity in the nitration of diphenyl carbonate to di(4-nitrophenyl) carbonate at -15.degree. C. to -20.degree. C. It would be of interest therefore to find a method for making higher purity di(4-nitrophenyl) compounds directly by improving the regioselectivity of the nitrating process.
Now it has been found that by carrying out the nitration process of diphenyl compounds in the presence of nitrobenzene, not only can the regioselectivity to the para isomer be increased, but the nitration can also be effected with the improved regioselectivity at essentially ambient temperature instead of the more usual near-ice temperatures used in earlier nitrations.